Polyphenylene sulfide (hereinafter abbreviated as PPS) resins have favorable properties as engineering plastics such as excellent heat resistance, barrier properties, chemicals resistance, electric insulation and wet heat resistance and are processed mainly by injection molding and extrusion molding into various electric and electronic parts, mechanical parts, automobile parts, films, fibers, etc.
However, since PPS resins are high in melting point, high temperatures are necessary for melt-processing them. Therefore, a volatile component is likely to be generated, and, for example, especially in the case where fibers are produced by melt spinning, there arises a problem that the volatile component contaminates the die, to cause yarn breaking. So, it is strongly desired to decrease the volatile component. In this invention, it has been found that thermal oxidation treatment under specific conditions can greatly decrease the volatile component, and a method for obtaining a granular or pelletized PPS resin greatly improved especially in melt spinnability has been found.
The thermal oxidation treatment of PPS resins has been performed hitherto. For example, Patent Document 1 discloses an extrusion molded article obtained by curing a PPS resin for keeping the polymer viscosity in a range from 5000 to 16000 poises (500 to 1600 Pa·s) (310° C., shear rate 200/sec) and keeping the non-Newtonian coefficient n in a range from 1.5 to 2.1, and melt-extruding the cured resin. However, a viscosity of 5000 poises corresponds to less than 100 g/10 min in terms of melt flow rate, and since the PPS resin is too high in melt viscosity, the pressure during spinning is too high. So, such a resin is unsuitable for melt spinning. Further, as disclosed in the patent document, the thermal oxidation treatment degree of the PPS resin is also relatively large, and if the thermal oxidation treatment degree is too large, there arises such a difficulty that a gelation product is likely to be produced for clogging the pack during melt spinning and that the pack pressure is likely to be sharply raised.
Patent Document 2 discloses a method for producing a PPS of 500 g/10 min or less in melt flow rate and 1/2 to 1/30 in the ratio of the melt flow rate after oxidation crosslinking to that before oxidation crosslinking, by oxidizing and crosslinking a PPS of 2000 g/10 min or less in melt flow rate. However, if such a high degree of thermal oxidation treatment as to achieve 1/2 to 1/30 as the ratio of the melt flow rate after oxidation crosslinking to that before oxidation crosslinking is applied, a gelation product is likely to be produced for clogging the pack during melt spinning, and the pack pressure is likely to be sharply raised. So, such a PPS resin is unsuitable for melting spinning, and in fact, Patent Document 2 does not describe the application of the PPS resin to fibers at all.
Patent Document 3 discloses a method for curing a PPS resin by oxidizing and crosslinking a PPS resin with a melt flow rate of 500 g/10 min or less before oxidation crosslinking to reach a melt flow rate of 100 g/10 min or less. However, since the PPS with a melt flow rate of 100 g/10 min or less is too high in melt viscosity, the pressure during spinning is too high. So, the PPS is unsuitable for melt spinning.
Patent Document 4 discloses a method of treating a granular PPS resin with a weight average molecular weight of 30,000 or more and an average particle diameter of 50 μm or less by thermal oxidation. However, as described in Patent Document 4, to obtain a PPS resin with a weight average molecular weight of 30,000 or more and an average particle diameter of 50 μm or less, a special polymerization reactor or grinding is necessary to raise the cost. So, it is not a general method. Further, such fine PPS particles cannot be smoothly fed into the extruder for melt kneading, and the amount of the PPS that can be melt-kneaded and extruded per unit time is small economically disadvantageously.
Patent Document 5 discloses a method for curing a PPS resin in a low oxygen atmosphere, but the melt viscosities of the PPS resins before thermal oxidation treatment disclosed in the examples are very low. Such PPS resins are low in melt spinnability. Even if such a PPS resin is thermally oxidized and crosslinked to be raised in viscosity to a range where melt spinning is possible, a gelation product is likely to be produced for clogging the pack during melt spinning since the thermal oxidation treatment is too strong, and the pack pressure is likely to be sharply raised. Further, high yarn strength cannot be exhibited either. In fact, Patent Document 5 does not describe the application of such a PPS resin to fibers at all.    Patent Document 1: JP63-207827A (claims)    Patent Document 2: JP62-197422A (claims)    Patent Document 3: JP5-43692A (claims)    Patent Document 4: JP6-248078A (claims)    Patent Document 5: JP1-121327A (claims)